Sodium chloride is separated from its solution in water primarily by the evaporation method. This technique is widely used because sodium chloride (salt) is a non-volatile solute, meaning it does not turn into a gas easily, while water is a volatile solvent and readily evaporates upon heating.
Understanding the Evaporation Method
The evaporation method relies on the difference in boiling points between the solvent (water) and the solute (sodium chloride). Water has a relatively low boiling point (100°C), allowing it to turn into vapor when heated, leaving behind the solid sodium chloride.
Principles Involved:
- Volatility Difference: Water evaporates, leaving the solid salt behind.
- Non-Volatile Solute: Sodium chloride does not evaporate at the temperatures required to boil water.
Step-by-Step Process of Separation
Separating sodium chloride from its aqueous solution using evaporation is a straightforward laboratory procedure, often demonstrated in Class 9 chemistry.
- Preparation of the Solution: Start with the sodium chloride solution (saltwater) in a suitable container, such as a beaker or an evaporating dish.
- Heating: Place the evaporating dish containing the salt solution on a wire gauze over a Bunsen burner or a hot plate.
- Evaporation: Gently heat the solution. As the water absorbs heat, its molecules gain kinetic energy and escape into the atmosphere as water vapor. It's crucial to heat gently to prevent the solution from spluttering, especially as it becomes more concentrated.
- Residue Formation: As the water continues to evaporate, the concentration of sodium chloride in the remaining solution increases. Eventually, all the water will evaporate, leaving solid sodium chloride crystals as a residue at the bottom of the evaporating dish.
- Cooling and Collection: Allow the evaporating dish and the deposited salt to cool down. Once cool, the pure sodium chloride can be scraped out and collected.
Practical Insights:
- Controlling Heat: Overheating can cause the salt to decrepitate (splutter and jump), leading to loss of product.
- Purity: This method yields relatively pure sodium chloride. However, if other non-volatile impurities were present in the original water or salt, they would also remain as residue.
- Water Recovery (Optional): If the aim is also to recover pure water, a distillation apparatus would be used, which combines evaporation with condensation. However, for just separating salt from water, simple evaporation is sufficient.
Why Evaporation is Suitable for Sodium Chloride
The suitability of the evaporation method for separating sodium chloride from its solution is due to several key characteristics:
- Non-Volatile Nature of Salt: Sodium chloride is a solid at room temperature and has a very high melting point (801°C) and boiling point (1413°C).
- Volatile Nature of Water: Water has a relatively low boiling point (100°C), making it easy to vaporize.
- Simple Setup: The method requires basic laboratory equipment, making it a common demonstration in schools.
- Effective Separation: It provides a clear separation of the solid solute from the liquid solvent.
This method highlights a fundamental concept in chemistry: different substances have different physical properties that can be exploited for separation.
